Already known from DE-OS 23 62 444 is an apparatus of the afore-mentioned type, on which the method of the indicated type is performed. In this respect, the moulding core extends quite a way in the production direction until it is in front of the nozzle gap. In the region of the expansion space formed by the frustoconically widening out moulding core and the moulding recess there are numerous fin moulding recesses, the filling of which takes place in an undefined manner. Furthermore, there is no guarantee of reliable transporting of the tube with fins through the moulding chamber.
Known from U.S. Pat. No. 3,998,579 is an apparatus similar to the afore-mentioned apparatus, in which the moulding chamber is adjacent a molten batch passage. Mounted at the end of the moulding chamber is a frustoconically widening moulding core by means of which the tube is calibrated. Here, too, filling of the fin moulding recesses is undefined. Furthermore, there is a considerable danger of molten material penetrating the region between the nozzle and the moulding recess and burning.